<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<?php
			echo "输出系统预定义常量：";
			echo "<br/>当前操作系统为：" . PHP_OS;
			echo "<br/>当前PHP版本为：" . PHP_VERSION;
			echo "<br/>当前文件路径为：" . __FILE__;
			
			echo "<br/>输出自定义常量：";
			define("PI", 3.14159); //定义常量PI
			$r = 20;
			$c = PI * $r * 2; //计算圆的周长
			echo "<br/>圆的半径=" . $r;
			echo "<br/>圆的周长=" . $c;
		?>
	</body>
</html>